I would also assume that MSN is better. Anyway, both have a common problem: they don't have a huge number of users like google adwords or yahoo.
if you are looking for sell your products , adwords would be the best choice ...For your question , my answer is Adcenter ...
I'm using all three, adwords, yahoo and adcenter. Adcenter has for my campaigns the best ratio, clicks and costs, vs. the other two. I get much higher impressions on g, but the costs are higher too. The best solution, use a coupon, open an account and try it with identical ad campaigns vs g or yah, and compare. Open account with $5, plus coupon, set budget at 45 for month and test..... puravida
I agree about adcenter but the trouble is that you potentially get very little volume so you need to weigh whether its even worth the trouble.